I recommend calling and asking to speak to the dean of admissions (or an assistant). Tell the your story, be forward, take responsibility, and apologize. I assume they'll either tell you it'll be okay or they'll tell you that it might change things. Once you get off of the phone, write an email thanking them for talking to you and attach the letter re-stating what you said on the phone.

I think you'd come off as being more responsible and serious than simply sending the email in.

That being said....That's a lot to disclose and it may hurt you--not necessarily for what it is, but because there might be a perception that you hid it intentionally... The only way to know, though, is to tell them and you'll only make it worse if you put off doing it.